This invention relates to a flat bed optical scanning beam deflection system, e.g., for use with a source of collimated light such as a laser, of the type wherein a beam of light, which is switched on and off as function of a pixel frequency, is projected by means of a rotating reflective element to create an image. More particularly, such prior art deflection systems are used as either an input scanner or an output scanner in various applications.
In a preferred application, the system of the present invention is employed in the typesetting of typographic characters by means of a light beam of parallel rays of light, i.e., a collimated beam, which is modulated as a function of the pixel frequency, and corresponding to image spots. In general, such flat bed deflection systems are employed to move or deflect a spatially fixed collimated light beam, hereinafter "light beam", in the direction of a scan line in a planar deflection plane which is perpendicular to the light beam, in a way such that independently from the position of the increment in the scan line, an equally long deflection path increment corresponds to each pixel period. In the flat bed deflection system of the type discussed above, which includes a rotatable or tiltable reflective member, this results in each angle of rotation corresponding to a predetermined proportional distance of scan in the deflection plane. Further, constant spacings are provided between consecutive scan lines in the perpendicular direction with respect to said scan lines.
Prior art flat bed laser scan or write systems consist of a laser, a beam expander (i.e. a telescopic system), a beam deflector in the form of a galvanometer with attached mirror, or a rotating reflecting polygon followed by a so called f.theta. objective. The latter objective has its entrance pupil on the deflecting mirror, and is designed with distortion so that the angular deflection of the scanning mirror is linearly proportional to the tangential movement of the beam in the image plane.
However, it has been found that a number of possibilities for error exists in such systems due to limitations with respect to manufacturing tolerances in the production of such flat bed deflection systems. One especially common cause of error is caused by the bearing on which the rotatable or tiltable face of the reflective member or element is supported. More particularly, when certain tolerances are exceeded in the manufacture of the bearing, it will tend to wobble causing what is known as polygon pyramidal error. In these systems, when the reflective member is a polygon having a plurality of reflective faces staggered relative to each other, such wobble will disturb the relationship between consecutive deflection lines, (i.e., the polygon pyramidal error will manifest itself as a result of the angular deviation of each reflective face of the polygon relative to a reference face thereof).
Accordingly, in order to effect the deflection in a manner such that the scan line is substantially precisely maintained within a single planar deflection plane, which is particularly desirable when the device is employed in typographic applications considering that in such application even minute irregularities will be noticeable, it becomes necessary to strive for a deflection angle which is as large as possible. More specifically, by increasing the deflection angle for a predetermined deflection angle, the optically effective distance or arm between the rotatable or tiltable reflective face and the deflection plane is correspondingly reduced. Thus, irregularities in the movement and position of the reflective face, for instance of a polygonal reflective member do not manifest themselves in as pronounced a manner within a particular deflection plane.
The exact positioning of an image spot along the lines in the image plane is also desirable in order to be able to determine the position of the image through the use of a conventional and relatively simple rotation sensor connected to the rotatable active reflective faces. Otherwise, extensive measurements through the use of a screen ruler in the image plane, and by measuring of the beam correspondingly deflected as a result of the modulated beam transferred through the associated optical system, which images a movable image spot of the measured beam on a photo receiver, becomes necessary.
In the prior art flat bed deflection systems, the angle of deflection is restricted within predetermined limits along a perpendicular line on the film plane because the image spot must not be imaged at too large a size since to do so will cause the image at the edges to become increasingly ellipse-shaped. In such typical prior art optical flat bed deflection systems, the optical system arranged between the rotatable or tiltable reflective face, and the deflection plane for deflecting a light beam through the various deflection positions in the deflection plane is comprised of a flat field lens in combination with multiple other members, resulting in an expensive arrangement. These lenses, which are employed in these conventional prior art systems, are also generally referred to as f.theta. lenses. It is however a disadvantage of these f.theta. lenses that they may be employed for use in situations where the deflection angle is highly limited in size, preferably in case where high resolution is desired, such as in a typographic typesetting apparatus. The entrance angle of such lens systems is also correspondingly highly limited in size.
In order to enlarge the entrance angle of such linearising optical systems, it has been previously suggested to provide an optical system which includes an aplanatic lens, i.e., a lens which is corrected for spherical aberration, including a first approximately planar face facing the rotating active reflective face and a second sperically-convex face facing the scan line plane, i.e., the image plane, such that the main beam will, at any angle of deflection, essentially impinge upon the lens as well as on a field flattening mirror near the deflection plane. Such a system is generally disclosed in German Patent Application No. P 34 04 407.8 whose disclosure is specifically incorporated by reference herein.